In WSN Gallery, amongst your switches at Admin Panel -> Settings -> Switches is an "image editing" option. This enables a row of tools beneath the images that allow your visitors to manipulate the images in your gallery. If you're using GD this will be just the basics like zooming in, negating and grayscaling... if using ImageMagick, a much wider array of options are present.

This is fun but largely pointless and wastes your server's CPU, so you may prefer to turn it off.